The catalytic dehydrogenation of hydrocarbons according to the formulaCnH2n+2CnH2n+H2,  (1)which is normally performed in the gas phase at a temperature of 450° C. to 820° C., is a highly endothermic equilibrium reaction, the reaction rate of which is limited thermodynamically and which depends on the respective partial pressures and temperature. The dehydrogenation reaction is favoured by low partial pressures of the hydrocarbons and by high temperatures.
The dehydrogenation reaction can be performed adiabatically or non-adiabatically or approximately isothermally. If the dehydrogenation is performed in an adiabatically operated catalyst bed, the endothermic reaction will cause the temperature to decrease over the length of the catalyst bed. The reaction rate in the catalyst bed is thus limited so that several catalyst beds are required to achieve the desired high reaction rates and re-heating is necessary downstream of each catalyst bed. In order to achieve reasonable reaction rates, several catalyst beds are normally arranged in series and the reaction system is re-heated downstream of each catalyst bed.
If the dehydrogenation is performed in a non-adiabatically operated catalyst bed, the catalyst bed can be heated in order to maintain a high temperature. Because of the fact that the temperature in the reaction system is kept constant, the reaction rates are appropriately high. Because of the location of the point of thermodynamic equilibrium, however, the disadvantage is that these high reaction rates can only be achieved at high temperatures, as a result of which the selectivity of olefin formation is reduced. Hence, consecutive reactions will increasingly take place, so that undesired products will form, such as CH4, CO, CO2, C2H4, C2H6 and coke.
The by-products thus formed, especially finely dispersed coke, precipitate in the course of the reaction on the catalyst, thus causing its state to change continually. The catalyst becomes coated with an undesired substance and is thus less accessible for the reactants. This means that the catalyst becomes deactivated. The activity of the catalyst for alkane dehydrogenation and the selectivity for alkene formation could deteriorate. This would result in deterioration of the efficiency of the process as a whole. Because of operational requirements, such a deactivation can only be tolerated up to certain limit, because an economically viable operation of the plant could no longer be guaranteed. In order, therefore, to counter-act a negative influence on the process, the catalyst has to be regenerated after a certain reaction period in order to recover its activity.
Depending on its characteristics, the catalyst is regenerated by bringing it in contact with an oxygen-bearing gas under conditions defined for the regeneration of the catalyst. The conditions for such a regeneration may differ from those required for the dehydrogenation. An oxygen-bearing gas diluted with steam is also often fed through the catalyst. As a result of this procedure, the by-products on the catalyst are reduced, with the result that the catalyst can regain its activity. If an oxygen-bearing gas diluted with steam is used for catalyst regeneration, the carbon-bearing deposit reacts to carbon dioxide as the main product. The carbon-bearing deposit is converted to gaseous products by this reaction and is removed from the system.
As the conditions for the alkane dehydrogenation process differ from the catalyst regeneration process, the alkane dehydrogenation process is interrupted after a certain period of operation and substituted by the catalyst regeneration process. Thereafter, the reactor bed is again available for dehydrogenation. Both these processes, i.e. the alkane dehydrogenation and catalyst regeneration, are thus performed periodically. In order to render the overall process economically efficient, this can take place in two or a plurality of catalyst beds, in which the reaction and regeneration processes are implemented in cyclic alternation. In order to ensure optimum catalyst regeneration, an optimum production and regeneration sequence should be adopted.
To optimise the plant, a plurality of catalyst lines is used, the dehydrogenation and regeneration processes being performed in cyclic alternation. Some of the catalyst lines can be used for alkane dehydrogenation, while other catalyst lines can simultaneously be regenerated by passing an oxygen-bearing gas or an oxygen-bearing gas diluted with steam over the catalyst. It is also possible to remove the catalyst deposits by a reductive process, although an oxidative process is normally faster and more effective. If a catalyst line has already completed the regeneration process before another catalyst line is ready for regeneration, it can be kept ready for use by continuing to pass an oxygen-free gas through the catalyst bed, thus ensuring that there is always a reaction line is always available.
Examples of such processes can be found in patent literature. DE 19858747 A1 describes a process for catalytic dehydrogenation of alkanes by non-adiabatically operated process. To prolong the dehydrogenation period, water vapour and hydrogen are admixed to the process gas. The dilution of the reaction mixture by these companion gases results in longer service lives of the catalyst, because the separated carbon-bearing deposits partially react with the companion gases to carbon monoxide and water and are removed from the process. After a certain reaction period, the catalyst is completely regenerated by interrupting the dehydrogenation and passing an oxygen-bearing gas through the catalyst bed. In a subsequent purging step after regeneration, hydrogen or a mixture of hydrogen and alkanes are fed through the catalyst bed for a reductive purification of the catalyst surface. These processes can be performed cyclically over several reactor lines.
U.S. Pat. No. 5,235,121 A describes a process for the catalytic dehydrogenation of alkanes in a non-adiabatic process. To regenerate the catalyst, an oxygen-bearing gas diluted with steam is used. In order to reduce heat losses, part of the gas used for regeneration and which is heated in said process is returned to dehydrogenation process. In a purging step following the regeneration, a hydrogen-rich reforming product is fed through the catalyst bed. These processes can be performed in cyclic alternation over a plurality of reactor lines. Preferred starting materials are alkanes with a C number of up to C12.
U.S. Pat. No. 4,229,609 A describes a process for the catalytic dehydrogenation of alkanes by a non-adiabatically operated process. To regenerate the catalyst, a consecutive sequence of purge gases is fed through the catalyst bed. The catalyst is first freed from alkane by feeding water-vapour-bearing gas through the catalyst, then regenerated by feeding oxygen-bearing gas through the bed and finally freed from oxygen-bearing regeneration gas by feeding water vapour through the bed. Admittedly, no durations are quoted for the individual regeneration phases. However, an experimental example (column 3) demonstrates the performance of a 30-minute dehydrogenation reaction, followed by a regeneration phase by a 1-minute purge with steam, a 28-minute phase during which an oxygen-bearing gas is fed through the catalyst and finally a 1-minute purge process using steam. The dehydrogenation and catalyst regeneration processes can be performed in cyclic alternation over several reactor lines. Preferred starting materials are alkanes with a C number of C6 to C9.
Irrespective of the mode of operation selected, it is found that that, despite regeneration, the catalyst deteriorates in the course of time with regard to the catalytic dehydrogenation reaction. Regeneration frequently takes place under conditions which adversely affect the catalyst properties. Thus, an oxygen-bearing gas is passed through the catalyst at an elevated temperature, as a result of which the catalyst properties may change. Due to the exothermal burn-off process, temperature control of the catalyst is not always a simple matter. The catalyst could therefore cake and its activity could deteriorate after a few cycles. The selectivity for the desired process of olefin formation could then be reduced.
The aim of the present invention is, therefore, to find regeneration conditions, under which the activity of the catalyst and the selectivity for the desired process of dehydrogenation is maintained even after a large number of regeneration cycles. The sought-after procedure should ideally influence catalyst activity and selectivity in a manner that will permit these parameters to be adapted optimally for the process. The regeneration process should maintain the catalyst activity at a constant level over as many cycles as possible and suppress the formation of carbon-bearing deposits.
Now, it was found that the task can achieved if the catalyst is regenerated by feeding oxygen-free and oxygen-bearing gases are fed through the catalyst bed in a certain temporal ratio. As a result of the mode of operation according to the invention, the activity of the catalyst for the conversion of alkanes is adjusted to a value suitable for the process. The selectivity for the desired dehydrogenation process is not only maintained by the mode of operation according to the invention, but also optimised to continually obtain, in total, a high yield of the desired alkene and to suppress the formation of by-products. In the regeneration process according to the invention, it is possible, to a large extent, to adjust the dehydrogenation equilibrium after feeding the regeneration gas through the catalyst bed even over many regeneration cycles. Thus, an improved mode of operation of the plant under economic aspects can, on the whole, be achieved.